I ordered a set of K66s from John at Moto Amore two weeks ago and had them installed Friday. My 250 was in for her 600-mile service and I chose to replace the stock Michelin Pilot Sports, as I don't see them as being suitable for winter driving here in MN.
First impressions, in good driving conditions: the Heidenaus don't want to turn or lean as readily as the Michelins. Now you might say, "... but turning is the whole point of an MP3!", and I mostly agree. It's not a bad thing: what it feels like is that the scoot wants to be upright, like it has springs on the sides of the tires or a gyroscopic compensator installed. With the Michelins I never got a sense that they preferred being upright vs. being at any other angle - they felt very "round" laterally. The Heidenaus definitely give you feedback and a preference, IMO. As someone planning to ride in less-than-ideal conditions, I'm okay with that. Staying upright means not falling!
As far as traction goes, they feel very grippy so far. I wonder if that's where some of the extra resistance to turning comes in: all that wonderful tread wants to keep a centerline and they don't flick as readily as the Michelins did. I haven't yet tried any twisties or low-speed maneuvers with them to see how they handle under more demanding conditions than a half-hour of urban street-crawling.
Some caveats: these are new-new tires on a nearly-new scoot with a nearly-new rider. As all three of these elements continue to "gain experience", I'll do my best to report back.